Compositions having improved tribological properties, methods of manufacture thereof and articles comprising the same

ABSTRACT

Disclosed herein is a composition comprising a crosslinked organic polymer; wherein the composition has a coefficient of friction that is in a range of ±30% of a coefficient of friction for a composition comprising the same organic polymer that is not crosslinked; and wherein the composition has a lower K factor than a K factor of the composition comprising the same organic polymer that is not crosslinked; the coefficient of friction and the K factor being measured in a thrust washer apparatus as per ASTM D-3702, where the counter stationary surface in the thrust washer apparatus comprises carbon steel having a Rockwell C hardness of 18 to 22 and a 12 to 16 micro-inch surface finish.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/329,676 filed Dec. 8, 2008, the content of which is incorporated byreference herein in its entirety.

BACKGROUND

This disclosure relates to compositions having improved tribologicalproperties, methods of manufacture thereof and to articles comprisingthe same.

Organic polymers are often used in applications involving thetransmission of force or energy (e.g., the transmission of torque, thetransmission of motion, and the like) where they are subjected tofriction. Friction brings about mechanical abrasion. It is desirable fororganic polymers to display as high abrasion resistance as possible.However, most polymers display poor abrasion resistance when subjectedto friction at moderate pressures and at moderate sliding velocitiesthat are generally employed in such frictional applications.

In order to reduce damage by mechanical abrasion, lubricants are oftenadded to the organic polymer. Lubricants decrease the coefficient offriction and in turn improve wear resistance. However, the addition oflubricants has several drawbacks. In particular, in applications whereforce or torque needs to be transmitted, a high coefficient of frictionis desirable because it allows for higher efficiency in the transmissionof these forces. Lubricants have the effect of decreasing thecoefficient of friction and therefore cannot be used in theseapplications. It is further desirable to have an organic polymericcomposition that can display resistance to catastrophic failure whenused in frictional applications that use high pressure and highvelocities.

SUMMARY

Disclosed herein is a composition comprising a crosslinked organicpolymer; wherein the composition has a coefficient of friction that isin a range of ±30% of a coefficient of friction for a compositioncomprising the same organic polymer that is not crosslinked; and whereinthe composition has a lower K factor than a K factor of the compositioncomprising the same organic polymer that is not crosslinked; thecoefficient of friction and the K factor being measured in a thrustwasher apparatus as per ASTM D-3702, where the counter stationarysurface in the thrust washer apparatus comprises carbon steel having aRockwell C hardness of 18 to 22 and a 12 to 16 micro-inch surfacefinish.

Disclosed herein too is a composition comprising a crosslinked organicpolymer; wherein the composition does not undergo catastrophic failurefor a period of about 1 to about 48 hours when tested in a thrust washerapparatus as per ASTM D-3702, where the counter stationary surface inthe thrust washer apparatus comprises carbon steel having a Rockwell Chardness of 18 to 22 and a 12 to 16 micro-inch surface finish, where thecomposition is subjected to a pressure of about 2 to about 400 poundsper square inch and a linear velocity of about 20 to about 400 feet perminute during the test in the thrust washer apparatus.

Disclosed herein too is a method comprising crosslinking an organicpolymer; the crosslinking being effective to produce a composition thathas a coefficient of friction that is similar to a coefficient offriction for a composition comprising the same organic polymer that isnot crosslinked; and wherein the composition has a lower K factor than aK factor of the composition comprising the same organic polymer that isnot crosslinked; the coefficient of friction and the K factor beingmeasured in a thrust washer apparatus as per ASTM D-3702, where thecounter stationary surface in the test equipment is comprises carbonsteel having a Rockwell C hardness of 18 to 22 and a 12 to 16 micro-inchsurface finish.

Disclosed herein too is a method comprising crosslinking an organicpolymer; the crosslinking being effective to produce a composition thatdoes not undergo catastrophic failure for a period of about 1 to about48 hours when tested in a thrust washer apparatus as per ASTM D-3702,where the counter stationary surface in the thrust washer apparatuscomprises carbon steel having a Rockwell C hardness of 18 to 22 and a 12to 16 micro-inch surface finish, where the composition is subjected to apressure of about 2 to about 400 pounds per square inch and a linearvelocity of about 20 to about 400 feet per minute during the test in thethrust washer apparatus.

Disclosed herein too is a method comprising crosslinking an organicpolymer; the crosslinking being effective to produce a composition thathas a coefficient of friction that is similar to a coefficient offriction for a composition comprising the same organic polymer that isnot crosslinked; and wherein the composition has a lower K factor than aK factor of the composition comprising the same organic polymer that isnot crosslinked; the coefficient of friction and the K factor beingmeasured in a thrust washer apparatus as per ASTM D-3702, where thecounter stationary surface in the test equipment is comprises carbonsteel having a Rockwell C hardness of 18 to 22 and a 12 to 16 micro-inchsurface finish; and subjecting the composition to friction.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a photograph showing the cracked non-crosslinked sample andthe crosslinked sample next to it with no visible damage.

DETAILED DESCRIPTION

The terms “first,” “second,” and the like, “primary,” “secondary,” andthe like, as used herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.

The terms “a” and “an” do not denote a limitation of quantity, butrather denote the presence of at least one of the referenced item.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are independently combinable with each other.

Disclosed herein are compositions that comprise crosslinked organicpolymers that can be used in frictional applications. Articlesmanufactured from the crosslinked organic polymers can resist highpressure and sliding velocities when compared with articles manufacturedfrom non-crosslinked organic polymers. The crosslinked organic polymersshow greater abrasion resistance than non cross-linked organic polymers.In an exemplary embodiment, the crosslinked organic polymers arepolyamides.

Articles manufactured the composition generally show a lower K factorthan a comparative composition that contains a non-crosslinked polymer.This generally results in improved wear performance and improved lifecycles for pieces of equipment that use the composition in frictionalapplications.

In one embodiment, the composition has a coefficient of friction that isin a range of ±30%, specifically in the range of ±20%, and specificallyin the range of ±10% of a coefficient of friction for a compositioncomprising the same organic polymer that is not crosslinked. Thecomposition generally has a K factor (also known as the “volumetric wearrate”, the “wear coefficient”, or the “abrasion factor”) than a K factorof the composition comprising the same organic polymer that is notcrosslinked; the coefficient of friction and the K factor both beingmeasured in a thrust washer apparatus as per ASTM D-3702, where thecounter stationary surface in the thrust washer apparatus comprisescarbon steel having a Rockwell C hardness of 18 to 22 and a 12 to 16micro-inch surface finish. In an exemplary embodiment, the compositionhas a coefficient of friction that is about the same as the coefficientof friction for a composition comprising the same organic polymer thatis not crosslinked, while at the same time displaying a lower K factorthan the composition comprising the same organic polymer that is notcrosslinked.

The K factor is given in Equation (1) below:

K=W/(F*D)  (1)

where W is the volume of material abraded from the test specimen, F isthe perpendicular load and D is the distance of sliding.

The composition is further advantageous in that an article manufacturedfrom the composition does not undergo catastrophic failure when testedin a thrust washer apparatus as per ASTM D-3702, when subjected to apressure of about 2 to about 400 pounds per square inch and a linearvelocity of about 20 to about 400 feet per minute for a period of about1 hour to about 48 hours. Catastrophic failure as defined herein is asudden inability to perform an operation that an article manufacturedfrom the composition hitherto could successfully perform.

In another embodiment, an article manufactured from the compositionperforms continuously for a period of about 1 to about 48 hours whentested in a thrust washer apparatus as per ASTM D-3702, where thecounter stationary surface in the thrust washer apparatus comprisescarbon steel having a Rockwell C hardness of 18 to 22 and a 12 to 16micro-inch surface finish and where the article is subjected to apressure of about 2 to about 400 pounds per square inch and a linearvelocity of about 20 to about 400 feet per minute during the test in thethrust washer apparatus.

In yet another embodiment, the article manufactured from the compositionperforms continuously for a period of about 16 to about 30 hours whenthe composition is subjected to a pressure of about 20 to about 80pounds per square inch and a linear velocity of about 40 to about 200feet per minute during the test in the aforementioned thrust washerapparatus as per ASTM D-3702. In yet another embodiment, the articlemanufactured from the composition performs continuously for a period ofabout 20 to about 28 hours when the composition is subjected to apressure of about 40 to about 60 pounds per square inch and a linearvelocity of about 100 to about 150 feet per minute during the test inthe aforementioned thrust washer apparatus as per ASTM D-3702.

As noted above the composition comprises crosslinked organic polymers.In one embodiment, it is generally desirable for the crosslinked organicpolymers to be below their glass transition temperatures at the initialtemperature of the frictional application. In one embodiment, it isgenerally desirable for the crosslinked organic polymers to be belowtheir melting temperatures at the initial temperature of the frictionalapplication. Examples of suitable organic polymers that can becrosslinked are blends of polymers, copolymers, terpolymers, orcombinations comprising at least one of the foregoing organic polymers.The organic polymer can also be an oligomer, a homopolymer, a copolymer,a block copolymer, an alternating block copolymer, a random polymer, arandom copolymer, a random block copolymer, a graft copolymer, a starblock copolymer, a dendrimer, or the like, or a combination comprisingat last one of the foregoing organic polymers prior to crosslinking.

Examples of organic polymers that can be crosslinked are polyacetals,polyolefins, polyacrylics, polycarbonates, polystyrenes, polyesters,polyamides, polyamideimides, polyarylates, polyarylsulfones,polyethersulfones, polyphenylene sulfides, polyvinyl chlorides,polysulfones, polyimides, polyetherimides, polytetrafluoroethylenes,polyetherketones, polyether etherketones, polyether ketone ketones,polybenzoxazoles, polyphthalides, polyacetals, polyanhydrides, polyvinylethers, polyvinyl thioethers, polyvinyl alcohols, polyvinyl ketones,polyvinyl halides, polyvinyl nitriles, polyvinyl esters, polysulfonates,polysulfides, polythioesters, polysulfones, polysulfonamides, polyureas,polyphosphazenes, polysilazanes, polysiloxanes, or the like, or acombination comprising at least one of the foregoing thermoplasticpolymers.

In an exemplary embodiment, the thermoplastic polymer is a polyamide ora copolyamide (e.g., a polyamideimide).

Examples of blends of polymers that can be crosslinked includeacrylonitrile-butadiene-styrene/nylon,polycarbonate/acrylonitrile-butadiene-styrene, acrylonitrile butadienestyrene/polyvinyl chloride, polyphenylene ether/polystyrene,polyphenylene ether/nylon, polysulfone/acrylonitrile-butadiene-styrene,polycarbonate/thermoplastic urethane, polycarbonate/polyethyleneterephthalate, polycarbonate/polybutylene terephthalate, thermoplasticelastomer alloys, nylon/elastomers, polyester/elastomers, polyethyleneterephthalate/polybutylene terephthalate, acetal/elastomer,styrene-maleic anhydride/acrylonitrile-butadiene-styrene, polyetheretherketone/polyethersulfone, polyether etherketone/polyetherimidepolyethylene/nylon, polyethylene/polyacetal, or the like.

As noted above, exemplary organic polymers that can be crosslinked toform the composition are polyamides. Polyamides are generally derivedfrom the polymerization of organic lactams having from 4 to 12 carbonatoms. Exemplary lactams are represented by the formula (I)

wherein n is about 3 to about 11. An exemplary lactam isepsilon-caprolactam having n equal to 5.

In one embodiment, the polyamide can be synthesized using anα,β-unsaturated gamma-lactone (such as 2(5H-furanone) to effect theregular, sequential alignment of side chains along a polyamide backboneas shown in the formula (II) below.

wherein n is about 50 to about 10,000, wherein each R is 1 to about 50carbon atoms and is optionally substituted with heteroatoms, oxygen,nitrogen, sulfur, or phosphorus and combinations thereof. Depending onthe side group (R), the method can produce many different types ofpolyamides. For instance, when R is a saturated long-chain alkyl group(such as when the amine is tetradecylamine), a polymer having alkylchains on one side of the polymer backbone and hydroxymethyl groups onthe other side of the backbone is formed. When the R group is apolyamine (such as pentaethylenehexamine), a polymer having aminosubstituted alkyl chains on one side of the polymer backbone andhydroxymethyl groups on the other side of the backbone is formed.

Polyamides may also be synthesized from amino acids having about 4 toabout 12 carbon atoms. Exemplary amino acids are represented by theformula (III)

wherein n is about 3 to about 11. An exemplary amino acid isepsilon-aminocaproic acid with n equal to about 5.

Polyamides may also be polymerized from aliphatic dicarboxylic acidshaving from about 4 to about 12 carbon atoms and aliphatic diamineshaving from about 2 to about 12 carbon atoms.

Exemplary aliphatic diamines are represented by the formula (IV)

H₂N—(CH₂)—NH₂  (IV)

wherein n is about 2 to about 12. An exemplary aliphatic diamine ishexamethylenediamine (H₂N(CH₂)₆NH₂). It is desirable for the molar ratioof the dicarboxylic acid to the diamine to be about 0.66 to about 1.5.In one embodiment, it is desirable to use molar ratios of about 0.81 toabout 1.22. In another embodiment, it is desirable to use molar ratiosof about 0.96 to about 1.04.

The dicarboxylic acids can be aliphatic dicarboxylic acids,cycloaliphatic dicarboxylic acids, or aromatic dicarboxylic acids.Examples of aliphatic dicarboxylic acids are aliphatic diacids thatinclude carboxylic acids having two carboxyl groups. Suitable examplesof cycloaliphatic acids include decahydro naphthalene dicarboxylic acid,norbornene dicarboxylic acids, bicyclo octane dicarboxylic acid,cis-1,4-cyclohexanedicarboxylic acid andtrans-1,4-cyclohexanedicarboxylic acids or the like, or a combinationcomprising at least one of the foregoing acids. Exemplary cycloaliphaticdiacids are cis-1,4-cyclohexanedicarboxylic acid andtrans-1,4-cyclohexanedicarboxylic acids. Examples of linear aliphaticdiacids are oxalic acid, malonic acid, pimelic acid, gluteric acid,suberic acid, succinic acid, adipic acid, dimethyl succinic acid,azelaic acid, or the like, or a combination comprising at least one ofthe foregoing acids. Examples of aromatic dicarboxylic acids areterephthalic acid, phthalic acid, isophthalic acid, naphthalenedicarboxylic acid, or the like, or a combination comprising at least oneof the foregoing dicarboxylic acids.

Exemplary polyamides comprise polypyrrolidone (nylon-4), polycaprolactam(nylon-6), polycapryllactam (nylon-8), polyhexamethylene adipamide(nylon-6,6), polyundecanolactam (nylon-11), polydodecanolactam(nylon-12), polyhexamethylene azelaiamide (nylon-6,9),polyhexamethylene, sebacamide (nylon-6,10), polyhexamethyleneisophthalamide (nylon-6,I), polyhexamethylene terephthalamide(nylon-6,T), polyamides of hexamethylene diamine and n-dodecanedioicacid (nylon-6,12), as well as polyamides resulting from terephthalicacid and/or isophthalic acid and trimethyl hexamethylene diamine,polyamides resulting from adipic acid and meta xylenediamines,polyamides resulting from adipic acid, azelaic acid and2,2-bis-(p-aminocyclohexyl)propane, polyamides resulting fromterephthalic acid and 4,4′-diamino-dicyclohexylmethane, and combinationscomprising one or more of the foregoing polyamides. The composition maycomprise two or more polyamides. For example the polyamide may comprisenylon-6 and nylon-6,6.

Copolymers of the foregoing polyamides are also suitable for use in thepractice of the present disclosure. Exemplary polyamide copolymerscomprise copolymers of hexamethylene adipamide/caprolactam(nylon-6,6/6), copolymers of caproamide/undecamide (nylon-6/11),copolymers of caproamide/dodecamide (nylon-6/12), copolymers ofhexamethylene adipamide/hexamethylene isophthalamide (nylon-6,6/6,I),copolymers of hexamethylene adipamide/hexamethylene terephthalamide(nylon-6,6/6,T), copolymers of hexamethylene adipamide/hexamethyleneazelaiamide (nylon-6,6/6,9), and combinations thereof.

Polyamides, as used herein, also comprise the toughened or super toughpolyamides. Generally, these super tough nylons are prepared by blendingone or more polyamide with one or more polymeric or copolymericelastomeric toughening agent. Suitable toughening agents can be straightchain or branched as well as graft polymers and copolymers, includingcore-shell graft copolymers, and are characterized as havingincorporated therein either by copolymerization or by grafting on thepreformed polymer, a monomer having functional and/or active or highlypolar groupings capable of interacting with or adhering to the polyamidematrix so as to enhance the toughness of the polyamide polymer.

The polyamides can be crosslinked by a number of different meansincluding thermally induced crosslinking, radiation inducedcrosslinking, or a combination comprising at least one of the foregoingmethods of crosslinking. An exemplary method of crosslinking involvesthe use of radiation-induced crosslinking. A crosslinking agent can beadded to the composition in order to facilitate crosslinking.Crosslinking accelerators, crosslinking initiators, crosslinkinginhibitors, or the like, can be added to the composition to control theamount of crosslinking. It is desirable for the crosslinking agent tocontain at least two functional groups capable of reacting with theamino groups of the polyamide.

The crosslinking agents can be a cyanurate crosslinking agent, anisocyanate crosslinking agent, a polyaldehyde crosslinking agent, aphosphine crosslinking agent, an epoxy crosslinking agent, or the like,or a combination comprising at least one of the foregoing crosslinkingagents.

Examples of cyanurate crosslinking agents are triallyl cyanurate,triallyl isocyanurate, or the like, or a combination comprising at leastone of the foregoing cyanurate crosslinking agents.

Suitable isocyanate crosslinking agents are monomeric or oligomericmolecules having 2 or more —N═C═O groups. Typically, the —N═C═O groupswill crosslink the polyamide between both hydroxyl (—OH) groups andamino (—NH₂ or —NH—) groups on the polyamide. Polyisocyanate compoundsuseful for crosslinking the polyamide include aliphatic and aromaticisocyanate compounds having an isocyanate functionality of at least 2.The polyisocyanate compounds can also contain other substituents whichdo not substantially adversely affect the reactivity of the —N═C═Ogroups during crosslinking of the polyamide. The polyisocyanate compoundcan also comprise mixtures of both aromatic and aliphatic isocyanatesand isocyanate compounds having both aliphatic and aromatic character.

Examples of polyisocyanate crosslinking agents include ethylenediisocyanate, ethylidene diisocyanate, propylene diisocyanate, butylenediisocyanate, hexamethylene diisocyanate, toluene diisocyanate,cyclopentylene-1,3,-diisocyanate, cyclohexylene-1,4-diisocyanate,cyclohexylene-1,2-diisocyanate, 4,4′-diphenylmethane diisocyanate,2,2-diphenylpropane 4,4′-diisocyanate, p-phenylene diisocyanate,m-phenylene diisocyanate, xylylene diisocyanate, 1,4-naphthalenediisocyanate, 1,5-naphthalene diisocyanate, diphenyl 4,4′-diisocyanate,azobenzene 4,4′-diisocyanate, diphenylsulphone 4,4′-diisocyanate,dichlorohexamethylene diisocyanate, furfurylidene diisocyanate,1-chlorobenzene-2,4-diisocyanate, 4,4′,4″-triisocyanatotriphenylmethane,1,3,5-triisocyanato-benzene, 2,4,6-triisocyanato-toluene,tetramethylxylene diisocyanate,poly((phenylisocyanate)-co-formaldehyde), or the like, or a combinationcomprising at least one of the foregoing polyisocyanate crosslinkingagents. An exemplary isocyanate ispoly(phenylisocyanate)co-formaldehyde).

The amount of polyisocyanate and the amount of polyamide used in thecrosslinking process can be varied depending upon the particularcrosslinking agent utilized, the reaction conditions and the particularproduct application contemplated. In one embodiment, the ratio of —N═C═Ogroups in the polyisocyanate to the total of amount of hydroxyl groupsand amino groups in the polyamide can be varied to achieve a desiredlevel of crosslinking. In one embodiment, at least 4 weight percent ofthe polyisocyanate to the total amount of polyamide will provideacceptable crosslinking. In one embodiment, enough polyisocyanate isadded to the polyamide such that an amount of up to about 30% of theavailable amino and hydroxyl groups in the polyamide are crosslinked bythe —N═C═O groups in the polyisocyanate.

The polyamides can be crosslinked using a polyaldehyde crosslinkingagent. Suitable polyaldehyde crosslinking agents are monomeric oroligomeric molecules having 2 or more —CHO groups. In one embodiment,the —CHO groups will crosslink the polyamide between amino groups on thepolyamide. Polyaldehyde compounds useful for crosslinking the polyamideinclude aliphatic and aromatic polyaldehyde compounds having apolyaldehyde functionality of at least 2. The polyaldehyde compound canalso comprise mixtures of both aromatic and aliphatic polyaldehydes andpolyaldehyde compounds having both aliphatic and aromatic character.Examples of polyaldehyde crosslinking agents include glutaraldehyde,glyoxal, succinaldehyde, 2,6-pyridenedicarboxaldehyde, 3-methylglutaraldehyde, or the like, or a combination comprising at least one ofthe foregoing polyaldehyde crosslinking agents.

In one embodiment, the ratio of —CHO groups in the polyaldehyde to thetotal of amount of amino groups in the polyamide can be varied toachieve a desired level of crosslinking. In one embodiment, thepercentage of polyaldehyde to the total amount of amino groups in thepolyamide is up to about 30% to provide acceptable crosslinking. Inanother embodiment, enough polyaldehyde is added to the polyamide suchthat an amount of up to about 30% of the available amino groups in thepolyamide are crosslinked by the —CHO groups in the polyaldehyde.

The polyamide can also be crosslinked using a phosphine crosslinkingagent having the general formula (A)₂P(B) and mixtures thereof, whereinA is hydroxyalkyl, and B is hydroxyalkyl, alkyl, or aryl and P isphosphorus. In one embodiment, the A groups will crosslink the polyamidebetween amino groups on the polyamide to form a Mannich base typelinkage —NH—CH₂—PRR₁, where R and R₁ are selected from hydroxy, methyl,hydroxyalkyl, alkyl and aryl groups.

Examples of phosphine crosslinking agents include tris(hydroxymethyl)phosphine, tris(1-hydroxyethyl)phosphine,tris(1-hydroxypropyl)phosphine, bis(hydroxymethyl)-alkylphosphine,bis(hydroxymethyl)-arylphospine, or the like, or a combinationcomprising at least one of the foregoing phosphine crosslinking agents.

In one embodiment, the ratio of A groups in the phosphine crosslinkingagent to the total of amount of amino groups in the polyamide can bevaried to achieve a desired level of crosslinking.

The polyamide can also be crosslinked using an epoxy crosslinking agentselected from epoxy resins having more than one epoxide group permolecule and mixtures thereof. An exemplary epoxy crosslinking agent isone having end groups of the formula (V):

the end groups being covalently bonded to carbon, oxygen, nitrogen,sulfur or phosphorus, or mixtures thereof. For example, R may bebisphenol-A. In general, the epoxy crosslinking agents will crosslinkthe polyamide between amino groups on the polyamide. The crosslinks areformed by attack at the epoxide rings by the amine proton, which opensthe epoxide ring forming an —OH group and forming a covalent crosslinkbetween the amine (or amide) and the terminal epoxide carbon.

Examples of epoxy crosslinking agents include polyglycidyl ethersobtainable by reaction of a compound containing at least two freealcoholic hydroxyl and/or phenolic hydroxyl groups per molecule withepichlorohydrin under alkaline conditions. These polyglycidyl ethers maybe made from acyclic alcohols, such as ethylene glycol, diethyleneglycol, and higher poly(oxyethylene) glycols; from cycloaliphaticalcohols, such as cyclohexanol and 1,2-cyclohexanediol; from alcoholshaving aromatic nuclei, such as N,N-bis(2-hydroxyethyl)aniline; frommononuclear phenols, such as resorcinol and hydroquinone; and frompolynuclear phenols, such as bis(4-hydroxyphenyl)methane,4,4′-dihydroxydiphenyl, bis(4-hydroxyphenyl) sulfone,1,1,2,2-tetrakis(4-hydroxyphenyl)ethane, and2,2,-bis(4-hydroxyphenyl)propane (otherwise known as bisphenol A). In anexemplary embodiment, the epoxy crosslinking agent is a bisphenol-Aglycidyl ether terminated resin. In general, the ratio of epoxide groupsin the epoxy crosslinking agent to the total of amount of amino groupsin the polyamide can be varied to achieve a desired level ofcrosslinking.

In one embodiment, the organic polymer may be crosslinked using any ofthe aforementioned crosslinking agents by blending the organic polymerwith the crosslinking agent. The blending can be dry blending, meltblending, solution blending or a combination comprising at least one ofthe foregoing forms of blending.

In solution blending, the organic polymer may be dissolved using asuitable solvent, followed by addition of the crosslinking agent andother desired accelerators, inhibitors, and the like to form a organicpolymer-crosslinking agent solution. Care is taken not to heat thissolution above the crosslinking temperature, as premature crosslinkingis undesirable. In one embodiment, the organic polymer-crosslinkingagent solution can be applied to a substrate and the substrate is heatedto complete the crosslinking process and create a crosslinked organicpolymer coating on the substrate.

In another embodiment, the solvent from the organic polymer-crosslinkingagent solution may be evaporated at a temperature below the crosslinkingtemperature, and the remaining organic polymer can be molded to form anarticle having a desired shape or geometry. It is desirable to mold theorganic polymer at a temperature below the crosslinking temperature ofthe organic polymer. The polymeric article may then be crosslinked bysubjecting it to heating or to radiation to bring about the desiredcrosslinking.

In yet another embodiment, the crosslinking agent can be added to theorganic polymer in a melt blending process. In this embodiment, theorganic polymer and the crosslinking agent can be dry blended to form aorganic polymer-crosslinking agent mixture in a device such as aHenschel mixer or a Waring blender prior to being fed to an extruder,where the mixture is melt blended. In another embodiment, a portion ofthe organic polymer can be premixed with the crosslinking agent to forma dry preblend. The dry preblend is then melt blended with the remainderof the organic polymer in an extruder. In one embodiment, some of theorganic polymer can be fed initially at the mouth of the extruder whilethe remaining portion of the organic polymer is fed through a portdownstream of the mouth.

Blending of the composition involves the use of shear force, extensionalforce, compressive force, ultrasonic energy, electromagnetic energy,thermal energy or combinations comprising at least one of the foregoingforces or forms of energy and is conducted in processing equipmentwherein the aforementioned forces are exerted by a single screw,multiple screws, intermeshing co-rotating or counter rotating screws,non-intermeshing co-rotating or counter rotating screws, reciprocatingscrews, screws with pins, barrels with pins, rolls, rams, helicalrotors, blades, or combinations comprising at least one of theforegoing.

Blending involving the aforementioned forces may be conducted inmachines such as single or multiple screw extruders, Buss kneader,Henschel, helicones, Ross mixer, Banbury, roll mills, molding machinessuch as injection molding machines, vacuum forming machines, blowmolding machine, or then like, or combinations comprising at least oneof the foregoing machines.

As noted above, the organic polymer can be crosslinked by subjecting itto radiation. The radiation may be ultraviolet radiation, electron beamradiation, x-ray radiation, alpha ray radiation, beta ray radiation,gamma ray radiation, or the like, or a combination comprising at leastone of the foregoing types of radiation.

The total amount of radiation can be about 10 to about 500 kiloGrays,specifically about 20 to about 450 kiloGrays, and more specificallyabout 70 to about 300 kiloGrays for an organic polymer having across-sectional area of about 100 square micrometers to about 900 squarecentimeters, specifically a cross-sectional area of about 200 squaremicrometers to about 800 square centimeters, specifically across-sectional area of about 300 square micrometers to about 100 squarecentimeters.

The compositions containing crosslinked organic polymers can beadvantageously used in frictional applications. In one embodiment,articles manufactured from the composition have a coefficient offriction at room temperature of greater than or equal to about 1.010,specifically greater than or equal to about 1.015, and more specificallygreater than or equal to about 1.020, and more specifically greater thanor equal to about 1.030 when measured as per ASTM D 3702, using a thrustwasher apparatus where the counter stationary surface in the testequipment is made of carbon steel having a Rockwell C hardness of 18 to22 and a 12 to 16 micro-inch surface finish.

In another embodiment, when subjected to a frictional application at apressure of about 2 pounds per square inch to about 400 pounds persquare inch and a velocity of about 20 feet per minute to about 400 feetper minute, an article manufactured from the composition has acoefficient of friction that is about 10% to about 70% greater,specifically about 20% to about 50% greater, and more specifically about25% to about 40% greater than a coefficient of friction for acomposition comprising the same organic polymer that is not crosslinked;the coefficient of friction being measured in a thrust washer apparatusas per ASTM D-3702, where the counter stationary surface in the testequipment is made of carbon steel having a Rockwell C hardness of 18 to22 and a 12 to 16 micro-inch surface finish.

In another embodiment, articles manufactured from the composition have aK factor of less than or equal to about 900·10⁻³ inch³/cal, specificallyless than or equal to about 600·10⁻³ inch³/cal, and more specificallyless than or equal to about 300·10⁻³ inch³/cal when measured as in athrust washer apparatus as per ASTM D-3702, where the counter stationarysurface in the test equipment is made of carbon steel having a RockwellC hardness of 18 to 22 and a 12 to 16 micro-inch surface finish.

In yet another embodiment, articles manufactured from the compositionwhen subjected to a frictional application at a pressure of about 2pounds per square inch to about 400 pounds per square inch and avelocity of about 20 feet per minute to about 400 feet per minute, havea K factor of less than or equal to about 6,900·10⁻³ inch³/cal,specifically less than or equal to about 6,600·10⁻³ inch³/cal,specifically less than or equal to about 6,300·10⁻³ inch³/cal, and morespecifically less than or equal to about 6,000·10⁻³ inch³/cal whenmeasured in a thrust washer apparatus as per ASTM D 3702, where thecounter stationary surface in the test equipment is comprises carbonsteel having a Rockwell C hardness of 18 to 22 and a 12 to 16 micro-inchsurface finish.

In one embodiment, an article manufactured from the composition has anincreased life cycle (while transferring an equivalent amount of torqueor energy) when compared with an article manufactured from a compositionthat contains non-crosslinked polymers. The life cycle for the articlecomprising the crosslinked organic polymer is increased by an amount ofgreater than or equal to about 5%, specifically greater than or equal toabout 10%, and more specifically greater than or equal to about 20% thanthe life cycle of an article manufactured from a comparative compositionthat comprises the same organic polymer in a non-crosslinkedcomposition.

In one embodiment, an article manufactured from the composition having alower abrasion rate (weight loss) when compared with an articlemanufactured from a comparative composition that comprises the sameorganic polymer in a non-crosslinked composition, can transmit a torquethat is in an amount of about 5% to about 50% greater, specificallyabout 10% to about 40% greater, and more specifically about 15% to about35% greater than the torque transferred by an article manufactured fromthe comparative composition.

In another embodiment, an article manufactured from the composition hasa lower abrasion rate (weight loss) when compared with an articlemanufactured from a comparative composition containing a lubricant, cantransmit a torque that is in an amount of about 5% to about 50% greater,specifically about 10% to about 40% greater, and more specifically about15% to about 35% greater than the torque transferred by an articlemanufactured from a comparative composition that comprises the sameorganic polymer in a non-crosslinked composition along with thelubricant (e.g., a polyolefin, a polyfluorocarbon, a polysiloxane, orthe like). The lubricant may be mixed with the organic polymer in thenon-crosslinked composition or can be reacted with the organic polymer(e.g., copolymerized) in the non-crosslinked composition.

As noted above, the articles manufactured from the composition performwell in frictional applications where high pressures and high velocitiesare desired. In one embodiment, the articles manufactured from thecomposition are advantageous in that they not undergo catastrophicfailure when tested in a thrust washer apparatus as per ASTM D-3702,when subjected to frictional applications that use a pressure of about 2to about 400 pounds per square inch and a linear velocity of about 20 toabout 400 feet per minute for about 1 to about 48 hours, specificallyabout 4 to about 30 hours and more specifically about 8 to about 24hours. Improving the ability of an organic polymer to withstand higherpressures, velocities and/or temperatures can in turn permit thematerial to be used in more demanding applications, where evenengineering thermoplastics cannot be used. It can be used to replacemetals, permitting weight reduction and allowing for higher designfreedom. Moreover, the ability to withstand higher pressures and/orvelocities allows for miniaturization of parts, and therefore furtherweight reduction and design freedom.

In another embodiment, the crosslinked polymers display a gradient inabrasion resistance from the outer crosslinked surface to an innercrosslinked surface. The crosslinked polymers generally display a highabrasion resistance at the outer surface and a lower abrasion resistanceat the lower surface.

The following examples, which are meant to be exemplary, not limiting,illustrate compositions and methods of manufacturing of some of thevarious embodiments of the friction resistant compositions comprisingcrosslinked organic polymers described herein.

EXAMPLES Example 1

This example was conducted to demonstrate the difference betweenfrictional properties of a crosslinked and non-crosslinked organicpolymeric sample. For this example, Nylon 6,6 was cross-linked viaexposure to beta radiation in an electron-beam process to form thecomposition. A cross-linking agent, i.e. triallyl-isocyanurate (TAIC)was melt mixed with Nylon 6,6. Exposure to radiation via cross-linkingwas performed on molded parts. The cross-linked parts were exposed to adose of 100 kiloGrays (kGy). The composition is shown in the Table 1.This composition was manufactured via melt mixing in a twin-screwextruder. The twin-screw extruder was a Clextral 21 having 9 barrels setat temperatures of 100, 280, 280, 280, 280, 280, 280, 280, and 280° C.The screw speed is 120 revolutions per minute. The extruder output is 10kilograms per hour.

The sample was injection molded to form a test sample. The injectionmolding machine had three zones set at temperatures of 260 to 280° C.(first zone), 265 to 285° C. (second zone) and 270 to 290° C. (thirdzone) respectively. The nozzle temperature was set at 275 to 295° C. Themelt temperature at the nozzle was 275 to 295° C. The mold temperaturewas 60 to 95° C. The compression ratio was 2:1 to 2.5:1. The screw was0.1 to 0.2 meter per seconds.

Tribological properties were tested using a thrust washer apparatus asper ASTM D-3702, the counter stationary surface being carbon steelhaving a 18 to 22 Rockwell C hardness and a 12 to 16 micro-inch surfacefinish. As noted above, the sample was crosslinked by subjecting it toradiation. TAIC was used as the crosslinking agent. A non-crosslinkedsample was used as a comparative sample. The abrasion tests wereconducted at a pressure (P) value of 40 pounds per square (psi) and alinear velocity (V) of 50 feet per minute (feet per minute) respectivelyand these did not change during the experiments. Results for theabrasion tests are shown later in Table 2.

TABLE 1 Composition (Commercial Name) Function and/or ChemicalDescription Wt % TECHNYL Nylon 6,6 80 27 AE1 ® TAICROS 5 ® Cross-linkingagent/triallyl-isocyanurate 5 IRGANOX Stabilizer and Primary Antioxidant0.05 1098 ® IRGAFOS Secondary Antioxidant 0.05 168 ® Sodium Mold releaseagent 0.1 Stearate BLACK Colorant 2 PEARL 800 ® R MOLY Masterbatch ofmolybdenum di-sulfide 5.7 CONC. 35% in Nylon 6,6 65% MILLED Milled Nylon6,6 7.1 DOMANID 33ABH ®

A comparison between crosslinked and non-crosslinked samples is shown inTable 2.

TABLE 2 Time Weight lost Interval Interval (hour) C.O.F. K (10⁻³inch³/cal) rate (10⁻¹ mg/hr) Crosslinked 1 70.6 1.055 995 4.5 2 95.30.983 288 1.3 3 119.4 1.029 377 1.7 4 142.2 1.139 410 1.8 5 166.3 1.021535 2.4 Non- 1 73.1 0.715 2616 11.7 crosslinked 2 101.4 0.735 1824 8.2 3120.5 0.738 1926 8.6 4 141.8 0.93 923 4.1 5 171.1 1.008 706 3.2

From the Table 2, it can be seen that the abrasion resistance of thecross-linked nylon 6, 6 is higher than the corresponding value of thenon-crosslinked Nylon 6,6. In fact, the weight-loss rate of thenon-crosslinked sample is, for each of the 5 intervals defined in thefirst column of Table 2, higher then the corresponding values measuredon the crosslinked sample. This is reflected in the corresponding Kfactors as well. It can also be seen that the difference between thecrosslinked and the non-crosslinked samples is higher at the beginningof the tests and becomes lower as the time for abrasion testingincreases.

However, in comparing the abrasion data relative for the last twointervals, i.e., 4 and 5, it can be seen that the non-crosslinked sampleshows a total abrasion rate of (4.1+3.2) 10⁻¹=7.3 10⁻¹ milligrams perhour (mg/hr), while the cross-linked sample shows abrasion rate of(1.8+2.4) 10⁻¹=4.2 10⁻¹ mg/hr, which corresponds to 42% reduction inwear.

It is also worth noting that the coefficient of friction (C.O.F.) of thecross-linked sample, for each of the 5-recorded intervals, is higherthan the corresponding values of the C.O.F. for the non-crosslinkedsample. Higher C.O.F. is beneficial in all applications where torque orforce needs to be transmitted.

Example 2

A second set of experiments was performed to test the materials' abilityto withstand conditions of high pressure (P) and velocity (V). As willbe seen in the Table 3, some of the conditions of pressure (P) andvelocity (V) for the Example 2 were greater than those of Example 1. Thetwo materials of Example 1—a crosslinked sample and a non-crosslinkedsample were tested for abrasion resistance at increasingly higherpressures and velocity. The test conditions and the results are shown inTable 3.

TABLE 3 Velocity Coef- Time Pressure (feet per ficient (h) (psi) minute)K (10⁻³ inch³/cal) of Friction Crosslinked 20.6 40 100 6660 0.934 25.960 100 7503 0.698 2 60 150 Machine Stopped Alarm 260° C. Non- 8 40 10010034 0.874 crosslinked 24.4 60 100 Part Cracked

A first step consisted in measuring both materials at P of 40 pounds persquare inch and V of 100 feet per minute, (this speed corresponds to thedouble of the speed used for the experiments reported in the Table 2).None of the two materials did show any failure. It is worth noting that,also at these P and V conditions, the k factor of the crosslinked sample(6.660 inch³/cal) was lower then the k factor of the non-crosslinkedsample (10.034 inch³/cal), which corresponds to a reduction of 34% dueto cross-linking.

When P was increased to 60 pounds per square inch (linear velocity (V)was kept at 100 feet per minute), the non-crosslinked sample failed,showing evident cracking, as can be seen in the FIG. 1, while thecross-linked sample did not show any evidence of failure after more the24 hours of testing. The FIG. 1 is a photograph showing the crackednon-crosslinked sample and the crosslinked sample next to it, with novisible damage. As can be seen in the FIG. 1, the crosslinked sampledoes not have any cracks. These results indicate that the cross-linkedsample can withstand higher P-V values than the non-crosslinked sample.The cross-linked sample therefore does not undergo the catastrophicfailure that the non-crosslinked sample undergoes.

The non-crosslinked ring failed when tested at P of 60 pounds per squareinch and V of 100 feet per minute. The cross-linked ring, on the otherhand, even after being tested at V of 150 feet per minute and P of 60pounds per square inch, shows no cracking. When tested at the mentionedP-V conditions, the temperature reached 260° C. This temperaturecorresponds to the safety alarm temperature of the testing equipment andto the melting point of Nylon 6,6. Even at these severe conditions oftemperature, pressure and velocity, the sample did not show anycracking.

As mentioned above, this data set indicates that crosslinking allows animprovement of the abrasion resistance even when the pressure andvelocity are increased during an abrasion resistance test. Improving theability of an organic polymer to withstand higher pressures, velocitiesand/or temperatures can in turn permit the material to be used in moredemanding applications, where even engineering thermoplastics cannot beused. It can be used to replace metals, permitting weight reduction andallowing for higher design freedom. Moreover, the ability to withstandhigher pressures and/or velocities allows for miniaturization of parts,and therefore further weight reduction and design freedom.

The tribological improvement illustrated above can be combined withother advantageous properties of the crosslinked organic polymers.Cross-linking reduces the corresponding coefficient of thermal expansion(CTE) of polymers when compared with non-crosslinked polymers. Thevalues for CTEs of polymers are around 40 to 60 ppm/° C., and aregenerally higher than the values for CTEs of metal (15 to 25 ppm/° C.).The high CTE of polymers limit their usage in applications where theycontact metallic parts. In the case of crosslinked polymers, theimproved tribological properties, combined with the lower CTE canincrease the possibility of replacing metal parts with crosslinkedorganic polymer parts thereby giving rise to a wider range ofapplications. Another advantage of crosslinking organic polymers is thereduced moisture uptake (when compared with non-crosslinked polymers)and therefore more stable performance under a variety of conditions ofexternal humidity.

In one embodiment, the CTE of the composition comprising the crosslinkedorganic polymer is up to about 10% lower, specifically up to about 20%lower, and more specifically up to about 50% lower than the CTE of acomposition that does not contain a crosslinked organic polymer. Inanother embodiment, the moisture uptake of the composition comprisingthe crosslinked organic polymer is reduced by an amount of up to about3%, specifically up to about 5%, and more specifically up to about 20%over a composition that does not contain a crosslinked organic polymer.

The composition containing the crosslinked polymer can alsoadvantageously be used in frictional applications where dimensionalstability is desired. For example, when the composition is used inmachine components such as gears, cams, worm wheels, piston rods, andthe like, the component does not suffer deformation during operationalincreases in temperature because of the crosslinking. The ability ofcrosslinked organic polymers to retain dimensional stability preventsthe seizing-up or sticking of machine parts to one another. For examplein a gear box, dimensional stability ensures that gears mesh with eachother and also ensure that the gear continues to rotate about is shaft(axis). This ability of crosslinked polymers to maintain dimensionalstability improves the life cycle of the machines and reduces downtimedue to maintenance of machine parts that is otherwise required.

While the invention has been described with reference to someembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing fromessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A composition comprising: a crosslinked organicpolymer; wherein the composition does not undergo catastrophic failurefor a period of about 1 to about 48 hours when tested in a thrust washerapparatus as per ASTM D-3702, where the counter stationary surface inthe thrust washer apparatus comprises carbon steel having a Rockwell Chardness of 18 to 22 and a 12 to 16 micro-inch surface finish, where thecomposition is subjected to a pressure of about 2 to about 400 poundsper square inch and a linear velocity of about 20 to about 400 feet perminute during the test in the thrust washer apparatus.
 2. Thecomposition of claim 1, where the composition does not undergocatastrophic failure for a period of about 16 to about 30 hours when thecomposition is subjected to a pressure of about 20 to about 80 poundsper square inch and a linear velocity of about 40 to about 200 feet perminute during the test in the thrust washer apparatus.
 3. Thecomposition of claim 1, where the composition does not undergocatastrophic failure for a period of about 20 to about 28 hours when thecomposition is subjected to a pressure of about 40 to about 60 poundsper square inch and a linear velocity of about 100 to about 150 feet perminute during the test in the thrust washer apparatus.
 4. Thecomposition of claim 1, where the crosslinked organic polymer is apolyamide.
 5. The composition of claim 1, where the composition iscrosslinked using a crosslinking agent.
 6. The composition of claim 1,where the composition is crosslinked using a cyanurate crosslinkingagent, an isocyanate crosslinking agent, a polyaldehyde crosslinkingagent, a phosphine crosslinking agent, an epoxy crosslinking agent, or acombination comprising at least one of the foregoing crosslinkingagents.
 7. The composition of claim 1, where the composition iscrosslinked using radiation.
 8. The composition of claim 7, where theradiation is xray radiation, electron beam radiation, gamma radiation,beta radiation, ultraviolet radiation, alpha radiation, or a combinationcomprising at least one of the foregoing forms of radiation.
 9. A methodcomprising: crosslinking an organic polymer; the crosslinking beingeffective to produce a composition that has a coefficient of frictionthat is similar to a coefficient of friction for a compositioncomprising the same organic polymer that is not crosslinked; and whereinthe composition has a lower K factor than a K factor of the compositioncomprising the same organic polymer that is not crosslinked; thecoefficient of friction and the K factor being measured in a thrustwasher apparatus as per ASTM D-3702, where the counter stationarysurface in the test equipment is comprises carbon steel having aRockwell C hardness of 18 to 22 and a 12 to 16 micro-inch surfacefinish.
 10. The method of claim 9, further comprising blending acrosslinking agent with the organic polymer prior to the crosslinking.11. The method of claim 9, further comprising heating the organicpolymer.
 12. The method of claim 9, further comprising irradiating theorganic polymer.
 13. The method of claim 12, where the irradiation isconducted with a radiation dosage of about 10 to about 500 kiloGrays perunit cross-sectional area of about 100 square micrometers to about 900square centimeters.
 14. A method comprising: crosslinking an organicpolymer; the crosslinking being effective to produce a composition thatdoes not undergo catastrophic failure for a period of about 1 to about48 hours when tested in a thrust washer apparatus as per ASTM D-3702,where the counter stationary surface in the thrust washer apparatuscomprises carbon steel having a Rockwell C hardness of 18 to 22 and a 12to 16 micro-inch surface finish, where the composition is subjected to apressure of about 2 to about 400 pounds per square inch and a linearvelocity of about 20 to about 400 feet per minute during the test in thethrust washer apparatus.
 15. The method of claim 14, further comprisingblending a crosslinking agent with the organic polymer prior to thecrosslinking.
 16. The method of claim 14, further comprising irradiatingthe organic polymer.
 17. The method of claim 14, where the irradiationis conducted with a radiation dosage of about 10 to about 500 kiloGraysper unit cross-sectional area of about 100 square micrometers to about900 square centimeters.
 18. A method comprising: crosslinking an organicpolymer; the crosslinking being effective to produce a composition thathas a coefficient of friction that is similar to a coefficient offriction for a composition comprising the same organic polymer that isnot crosslinked; and wherein the composition has a lower K factor than aK factor of the composition comprising the same organic polymer that isnot crosslinked; the coefficient of friction and the K factor beingmeasured in a thrust washer apparatus as per ASTM D-3702, where thecounter stationary surface in the test equipment is comprises carbonsteel having a Rockwell C hardness of 18 to 22 and a 12 to 16 micro-inchsurface finish; and subjecting the composition to friction.
 19. Themethod of claim 18, where the friction comprises sliding friction. 20.The method of claim 18, where the friction comprises static friction.